Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its sensitivity to octane-enhancing additives.
Industrial alkylation processes have historically used hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. The sulfuric acid alkylation reaction is particularly sensitive to temperature, with low temperatures being favored to minimize the side reaction of olefin polymerization. Acid strength in these liquid acid catalyzed alkylation processes is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. The hydrofluoric acid process is less temperature sensitive and the acid is easily recovered and purified.
U.S. Pat. No. 4,795,728 to Kocal, for example, teaches a hydrofluoric acid catalyzed alkylation process for producing motor fuel. The hydrofluoric acid catalyst complex includes from 0.5 to 5 weight percent of a cationic or anionic surfactant component enabling the process to be operated at an olefin:acid volumetric feed ratio of greater than 1.0 while maintaining acceptable alkylate quality.
Both hydrofluoric and sulfuric acid catalysts are gradually depleted in continuous alkylation processes and must be regenerated or replenished by mixture with fresh acid to maintain acid strength, reaction rate, and the resulting alkylate quality. Specifically, alkylate quality responds directly to increasing acid strength, and the acid makeup or regeneration rate is typically controlled together with other process variables such as temperature and space velocity, to meet a required alkylate quality specification.
Both sulfuric acid and hydrofluoric acid alkylation share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. Research efforts have been directed to developing alkylation catalysts which are equally as effective as sulfuric or hydrofluoric acids but which avoid many of the problems associated with these two acids. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
Catalyst complexes comprising BF.sub.3 as well as BF.sub.3 :H.sub.3 PO.sub.4 adducts have been proposed, and are discussed in greater detail below. While these catalysts effectively overcome the safety and environmental drawbacks of sulfuric and hydrofluoric acid alkylation systems, the volume and quality of BF.sub.3 alkylates have not, in the past, proven comparable to that of sulfuric or hydrofluoric acid alkylates.
U.K. Patent 545,441, assigned to Standard Oil Development Company, teaches a BF.sub.3 :H.sub.3 PO.sub.4 catalyzed isoparaffin-olefin alkylation process.
U.S. Pat. No. 2,345,095 to Bruner teaches a paraffin-olefin alkylation process catalyzed by a boron trifluoride-water complex, represented by the formula BF.sub.3 :nH.sub.2 O, where n is preferably from 1 to 1.5. The Bruner reference notes at page 2, left hand column, lines 13-23, that the BF.sub.3 :H.sub.2 O catalyst complex behaves similarly to sulfuric acid but is a superior alkylation catalyst because BF.sub.3 :H.sub.2 O does not promote oxidation to undesired byproducts.
U.S. Pat. Nos. 2,296,370 and 2,296,371 to Slotterbeck disclose a BF.sub.3 :H.sub.2 O:HF catalyst system and an isoparaffin-olefin alkylation process employing the same. The catalyst system is said to avoid yield loss due to oxidation of the resulting alkylate product. The Slotterbeck '370 and '371 patents also discuss loss of catalytic activity due to diminishing acid strength; see the Slotterbeck '370 patent at page 2, right hand column at line 75 through page 3, left hand column at line 55, and the the Slotterbeck '371 patent at page 2, right hand column at line 66, through page 3, left hand column at line 41,
U.K. Patent 550,711 teaches a process for increasing the activity of at least partially spent BF.sub.3 :H.sub.2 O catalyst systems for reuse in an organic condensation reaction. Briefly, the process volatilizes BF.sub.3 from the liquid catalyst mass to the extent required to promote separation of a distinct hydrocarbon phase from the catalyst mass. This hydrocarbon phase is then decanted off and fresh BF.sub.3 is added to restore catalytic activity.
Canadian Patent 424,000 teaches a process for producing gasoline boiling range hydrocarbons from isobutane and a normally gaseous olefin by absorbing the olefin in phosphoric acid of at least 75 weight percent concentration with an amount of isobutane equal to at least three moles of isobutane per mole of alkyl phosphate in the presence of a catalytic mixture of phosphoric acid and boron halide at temperature between 20.degree. C. and 60.degree. C.
U.S. Pat. No. 3,873,634 to Hoffman teaches a method of increasing the rate of ethylene alkylation by isobutane by carrying out the reaction simultaneously with the alkylation of a small amount of a higher weight olefin in the presence of a BF.sub.3 :H.sub.3 PO.sub.4 catalyst complex at low temperature and pressure.
U.S. Pat. No. 3,925,500 to Wentzheimer discloses a combined acid alkylation and thermal cracking process employing a BF.sub.3 :H.sub.3 PO.sub.4 acid catalyst in which unconverted propane and ethane from the alkylation process are converted, for example, to propylene and ethylene which are subsequently alkylated with isobutane to evolve a valuable liquid fuel.
The problem of acid consumption remains as an obstacle to commercialization of Lewis acid catalyzed alkylation, first because of the high cost of suitable Lewis acids, and second because of the cost of disposing of an acid neutralization byproduct if the acid is not recycled. Specifically, it would be highly beneficial to avoiding the capital and operating costs associated with a Lewis acid/light aliphatic fractionation section. For example, in a typical BF.sub.3 -catalyzed isobutane:butene alkylation process employing feeds containing minor amounts of lighter hydrocarbons, the reactor effluent product is first fractionated to separate C.sub.4 - components from C.sub.5 + alkylate. The overhead stream from this fractionation step typically contains not only BF.sub.3 but also unreacted propane which must be removed before the BF.sub.3 is recycled to the alkylation reactor. While the two components are separable by distillation, their close boiling points require a tall, expensive distillation tower. Further, the tower must be protected from corrosive attack by BF.sub.3, for example, by the use of nickel-rich alloys such as Monel.
Moreover, residual BF.sub.3 carried over into the product fractionation section reacts to form nonvolatile boron hydrates which precipitate out and form insoluble deposits inside the distillation columns and their associated reboilers. U.S. Pat. No. 3,631,122 reports that these deposits cause an undesirable deterioration in distillation column performance, and discloses a process for reacting these hydrates and removing the reaction product from the system. But clearly, it would be even more beneficial to preventing carryover of BF.sub.3 into the product fractionation section to avoid formation of these insoluble deposits.
U.S. Pat. No. 4,384,161 teaches a process of alkylating isoparaffins with olefins in the presence of a catalyst comprising a large pore zeolite capable of absorbing 2,2,4-trimethylpentane, e.g., ZSM-4, ZSM-20, ZSM-3, ZSM-18, zeolite Beta, faujasite, mordenite, zeolite Y and the rare earth metal-containing forms of zeolite Y, and a Lewis acid such as BF.sub.3, SbF.sub.5, or AlCl.sub.3. The use of a large pore zeolite in combination with a Lewis acid is shown in the '161 patent to increase the activity and selectivity of the zeolite thereby effecting alkylation with high olefin space velocity and low isoparaffin:olefin ratio.
Allowed U.S. patent application Ser. No. 425,497, filed Oct. 17, 1989, which is a continuation of U.S. patent application Ser. No. 219,130, filed Jul. 15, 1988, now abandoned, discloses heterogeneous isoparaffin:olefin alkylation in the presence of a large-pore zeolite, a Lewis acid, and water. The Lewis acid is suitably present in an amount which exceeds that required to saturate zeolite as well as any binder or matrix material which may be present.
Alkylate gasolines, such as those produced by the processes discussed above, are rich in isoparaffins and contain essentially no sulfur and aromatics. Generally exhibiting strong sensitivity to octane-enhancing additives, alkylate gasolines are prime candidates for blending into motor gasolines to meet the increasingly stringent environmental regulations restricting gasoline vapor pressure and aromatics content. However, none of these alkylate gasolines is suitable for use as blending stocks unless they are free from the acid components of the alkylation catalyst. Specifically, the Lewis acid component of the alkylation catalyst complexes employed in the processes discussed above must be removed before the alkylate product can be blended into gasoline.
Once-through BF.sub.3 operation together with product deacidification, if it had been previously contemplated, would have been eliminated from serious consideration due to prohibitive costs both the makeup Lewis acid as well as for disposal of the neutralization byproduct. Separating the Lewis acid and recycling the purified acid, on the other hand, reduces acid consumption but requires a substantial capital investment for fractionation process equipment, as noted above. Thus it would be desirable to provide a Lewis acid catalyzed alkylation process which achieves the alkylate quality of previously known but less environmentally acceptable processes while at the same time producing an alkylate product stream containing sufficiently low levels of Lewis acid that the product stream could be ecomonically deacidified, thereby affording once-through BF.sub.3 operation while avoiding the expense associated with Lewis acid recovery and recycle.